High-frequency apparatus



pt- 1953 J. R. WQODYARD 2,653,271

HIGH-FREQUENCY APPARATUS Filed Feb. 5, 1949 3 Sheets-Sheet l A/ OSC/L L4TOR Fig. 341.

Fig. 36.

IN V EN TOR. JOHN R W000 YARD Sept. 22, 1953 J. R. WOODYARD 2,653,271

HIGH-FREQUENCY APPARATUS Filed Feb. 5, 1949 s Sheets-Sheet 2 INVENTOR.(/OH/V f1. W000 WW0 Z4 TTORY.

5 Sheets-Sheet 3 INVENTOR.

,4 TTOR/VE )4 J. R. WOODYARD HIGH-FREQUENCY APPARATUS Sept. 22, 1953Filed Feb. 5, 1949 (/0///\/ A. VVOODYARD PULSE GENER/JTOR Patented Sept.22, 195? HIGH-FREQUENCY APPARATUS John R. Woodyard, Berkeley, Calif.,assignor to The Sperry Corporation, a corporation of DelawareApplication February 5, 1949, Serial No. 74,837

8 Claims. (01. 315-) The present invention relates to high frequencyapparatus and methods for producing high-energy electrically chargedparticles, and, more particularly, the invention is concerned with suchapparatus and methods wherein the electric fields contained withinsections of electromagnetic wave guides and/or cavity resonator devicesare utilized to energize the discrete particles of a beam of particles.

A principal object of the present invention is to provide new andimproved apparatus and. methods for producing a high-energy beam ofelectrically charged particles.

Another object of the present invention is to provide an arrangement forproducing a beam of electrons having an energy content of the order of abillion electron-volts useful in medical therapy or for the productionof highly penetrating X-rays or for other purposes.

Heretofore there have been employed diverse types of apparatus forproducing beams of highenergy charged particles. The known types ofapparatus may be divided, for the present discussion, into two generalclasses, e. g. (1) roundand-round or curvilinear accelerators, and (2)linear accelerators.

The first of the above-named classes includes that form of apparatus inwhich either a static or a variable magnetic field is employed, with orwithout accompanying electric fields, to whirl electrically chargedparticles in curved trajectories whereby a progressively increasingamount of energy is absorbed by the charged particles.

This class of acceleration apparatus has proved to be generallysatisfactory for the production of electrons of energies up to aboutelectronvolts. However, it may be shown that, following the teachings ofthe prior methods, the production of a beam of electrons having energiesabout ten times that of the presently attainable energies necessitateselectromagnets having pole pieces roughly 80 feet in diameter. Clearly,apparatus of this type is difiicult to construct and very expensive forthe production of billion-electronvolt beams of electrons.

A further object of this invention, therefore, is to provide a new andimproved electron energizing arrangement of the linear type whereby theneed for large electromagnets or other sources of intense magneticfields is substantially eliminated.

The linear types ofaccelerators heretofore utilized for producingstreams of electrically charged particles stemmed originally from theearly forms of cathode ray tubes, wherein electrons were continuouslyaccelerated in a unidirectional electric field maintained between acathode and an anode. Due to inherent limitations, the maximum energiesobtainable from such unidirectional fields were of the order of a fewmillion electron-volts. To extend the range of the obtainable energies,alternating field types of linear accelerators were used, amployingmeans for intermittently applying accelerating impulses to theparticles.

In one form, the electrons of the beam were exposed to the alternatingfield for a time approximately equal to the time of a half-cycle. Inanother form, the electrons were acted on by the field a plurality oftimes, but such accelerating impulses were supplied only during thepositive half-cycles of the accelerating field, the particles beingshielded from the field during the negative half-cycles so that onlyhalf the energy supplied to the field was constructively utilized. Latertypes of alternating current linear accelerators, of both these latterforms, extended the limit of the obtainable energy by using highfrequency electric fields provided by tuned circuits of the lumpedparameter type, and, later still, in the distributed parameter type ofcircuit embodied in cavity resonator apparatus.

An improved electron energizing apparatus is provided by the presentinvention whereby energies of the order of a billion electron volts areobtained which is, at least, ten times higher than the level previouslyobtained by prior known means. This high level of electron energy isrealized by the projection of an axial electron beam through acylindrical cavity resonator so excited as to support a standingelectromagnetic field whose configuration defines a zero magnetic fieldin the axial direction and an axial electric field characterized by alarge number of halfcycle variations.

The spatial separation of successive half-cycle variations is adjustedto be approximately equal to the free-space half-wavelength at theoperating frequency, so that when a beam of electrons, preliminarilyaccelerated to velocities nearly equal to that of light, is projectedaxially along the resonator axis, these high velocity electrons traversethe space between successive half-cycle variations in field insubstantially the same time that is required for the variations in theelectric field to occur. Consequently, a significant portion of theelectrons encounter electric fields favorably disposed for impartingenergizing impulses to the beam. Hence, after the beam has beensubjected to a predetermined high number 3 of such energizing impulses,corresponding to the number of half-cycle variations of the axialelectric field, the energy level of the electrons of the beam reacheshigh values such as electronvolts.

Accordingly, the present invention has for another of its objects theprovision oi an improved electron energizing apparatus of the lineartype wherein cavity resonator means are excited in such a manner as tosubject a beam of high ve-. locity electrons projected. therethrough inan axial direction to a plurality of energizing inrpulses during asingle transit of the beam through the resonator. I h

A feature of this invention lies in providing a linear type of electronenergizing apparatus with a cavity resonator excited in a resonant modesuch that its standing wave pattern defines a zero magnetic field intheaxial direction and anaxial electric field characterized by a plurality'of half-cycle variations and an arrangement within 'said resonator foradjusting the spatial separation of successive half-cycle variations toa value substantially equal to the frees'pa ce halt-wavelength at theoperating freq en y- Another feature of -the present invention is toprovide, in an electron energizing arrangement of the above descibedcharacter, a cylindrical resonator means characterized by the fact thatthe ratio of the length of resonator to its diameter of cross-section isvery high.

'A further object of the present invention consists of providing amethod of exciting a cavity resonator apparatus whose length is long as'eompared to its diameter which comprises the step of exciting saidcavity resonator in a high order axial electric mode of order (0, 1, n)where 'n is'an integer greater than zero. 7

A specific feature of the present invention lies/in the provision of aplurality of constriotions formed interiorly of'electromagneticwaveenergy guides-or cavity resonators to reduce theinterne'dal'spacings of the electromagnetic field variations.

'rhe'onsmccdns may hav'e a spatial separation equal to free space halfwavelength at the "operating frequency, or alternatively conitiriuousloadingfmay be employed by placing a plurality of constrictions alongthe length of the resonato per half-cycle variation in the 4 standingwave pattern.

additional object'of thepresent invention is to provide a device for theproduction of highenergy electricallyfcharged particles, wherein therequisite high-frequency excitation energy is impulsively supplied tothe device.

.A'still further object is to'provide means within a Wave-energy guidefor increasing the velocity of relatively slow-injected electrons sothat the'yliiay then be acted upon by the lectro ma'grieticfieldto'provide a high-energy beam of arm 'Still another "object of :thepresent'invention is to provide an arrangement 'for producinghigh-energy electron beams as above, wherein means "are I provided forsymmetrically coupling highfrequency electromagnetic energy from asource to the utilization device wherein coupling is effected atthe'entrance terminal of the device.

Yet another object of this invention is to provide "a symmetricalcoupling for high-energy 4 citation energy is effected at the exitterminal of the device.

A still further object of the present inven tion is to provide anarrangement for coupling high-frequency electromagnetic energy to autilization device having means for distributively coupling theresonator-excitation energy to the resonator so that the coupling isaccomplished substantially uniiormly'alohg the length of the cavityresonator.

Another object of the invention is to provide a novel couplingarrangement for a utilization device employed for the production of ahigh- ;energy electron beam which comprises a combii i'e'olsymmetrical;distributive coupling arrangement whereby undesired modes ofoscillation are substantially eliminated.

lhe invention also relates to the novel features or principles of theinstrumentalities described herein, whether or not such are used for thestated objects, or in the stated fields or cembinations. k

Other objects and advantages will beceriie apparent from thespecification taken in connection with the accompanying drawings,Where'- in the invention is embodied in concrete form.

In the drawings,

Fig. l is a diagrammatic representation of the electric fielddistribution within a metal cylinder;

Fig. 2 is a longitudinal cross sectional View of one embodiment of thepresent. invention together with a schematic showing of the 'e'nergizing circuit associated therewith;

Figs. 3c and 3b are explanatory graphs of voltages existinga't variouspoints of the "circuit of Fig.2.

Fig. l is a similar view of another modification of a portion of theinterior construction of the resonator of Fig.2;

1 Fig. 5 is a longitudinal cross sectional view of a modified form ofthe device shown in Fig. 2 in which the excitation energy is coupled tothe resonator at the entrance terminal thereof;

Fig. 6 is a similar view showing the coupling of the excitation energyat the exit terminal'oi the resonator;

Fig. '7 is a longitudinal cross sectional view of a modified embodimentor the present'invention in which the "excitation energy iscoupied tothe resonator at a plurality of points uniformly distributed along thelength of the resonator;

Fig. 8 'is acros'ssectional view taken along the line'88 or Fig. 7;

Fig. 9 is amodifi'c'ationof the arrangement shown in Fig. 7 and shown inlongitudi'nal cross Similar characters .of reference are used throughoutthe figures to indicate corresponding parts.

Referring now' to Fig. 1 ofthe drawing, which is a diagrammaticrepresentationof the instantaneous electromagnetic field distribution 0fa wave propagated in *a wave guide, the reference numeral 2 idesign-ates a hollow conducting 'cylindrical tube throughWhichielectroma'gnetic waves are regardedas being transmitted in 1a modeof propagation commonly indicated as TMo,1=. The distributions of theelectric and magnetic fields are here shown as lines 22 (correspondingto the electric field distribution) and the circles 24 and dots 26(corresponding to the magnetic field distribution, the circles 24indicating lines of magnetic force directed toward the reader and thedots 26 indicating lines of magnetic force directed away from thereader).

It is apparent that this mode of propagation is ideal for acceleratingelectrons or other charged particles for, if a beam of such particles isprojected axially along the tube 2| with a velocity such that theparticles in the beam traverse a distance 3 between the transverseplanes indicated by the dashed lines p and q corresponding to regions ofzero axial electric field, in a time equal to a half-cycle of thefrequency of the energy, then clearly, the electrons, assuming that theyare introduced into the guide in phase with the electrical field, willalways experience a field favorable for abstraction of energy therefrom.

When a wave guide, such as tube 2!, is closed at both ends there resultsone form of a resonator. Energy which is propagated through such aresonator encounters, in the first instance,

one of the conducting end walls interposed in its path, whereupon arefiected wave is set up which travels in an opposite direction. As aresult, the incident and reflected waves give rise to a standing wavepattern. In such case, although the electric and magnetic fieldintensities of the incident wave are in time and space phase coincidenceas are the electric and magnetic field intensities of the reflectedwave, the resultant electric and magnetic field intensities are in timeand space quadrature. From this it is apparent that when electromagneticenergy is propagated in a wave guide terminated in its characteristicimpedance, the maxima of the electric and magnetic field intensityvariations occur at the same points, as is evidenced by the arrangementof the lines of force of Fig. 1. On the other hand, where the guide isoperating as a resonator a 90 spatial separation of the respectivemaxima of electric and magnetic lines of force diagrammaticallyrepresents the field distribution.

Now, it is also known that if the tube 2| be closed by a pair ofconducting walls perpendicularly disposed with respect to the axis ofthe tube 2!, one of the walls being placed at the plane 1) and the otherat q, a high-Q cavity resonator is thereby formed having a resonant modeof oscillation known as the TMo,1,1 mode. Consequently, in consideringthe wave guide of Fig. 1 modified by the proposed conducting wallspositioned at planes p and q, the location of the lines of force will bealtered in the manner described above.

It is further known that a resonator may be constructed of such lengththat a plurality of half-cycle variations of field intensity may bepresent normal to the transverse field. In other words, in conventionalsymbolism the configuration of the field may be defined as the TMo,1,nmode, where n denotes the number of such half-wave patterns. As has beenindicated in the prior art, this mode of operation is ideal for theaccelerating or energizing of electrons.

A simple numerical example illustrates the nature of the problemsencountered and the limits thereby established in prior art devices.

6 energies of 10 electron-volts by means of a cylindrical cavityresonator excited in the TMo,1,o mode; operations in such a mode wouldnecessitate very high power of excitation. With such magnitude ofexcitation energy field emission or so-called low-temperature emissionassumes an important role in the successful maintenance of the standingwave pattern within the resonator. Electron emission that occurs mainlyas a result of a strong external field effect on the work function hasbeen frequently described as field emission. As it is well known, fieldemission imposes an upper limit on the voltage gradient which can besupported in an evacuated chamber before breakdown. This limit mayconservatively be placed at about 10 volts per centimeter. Thus, aresonator so excited and energized for the production of 10electron-volt particles would necessarily be about 10 centimeters orabout 30 feet long.

Theoretically, a cylindrical resonator excited in the TM0,1,0 mode maybe expanded to such a length since the resonant wavelength isindependent of the length of the chamber. However, to be an efficientlinear accelerator, it is necessary that the electrons traverse thelength of the resonator in a period corresponding to that of ahalf-cycle of the excitation frequency, so that the electrons may have amaximum net energy gain. To accomplish this, the resonator wouldnecessarily be operated at a wavelength approximately equal to 60 feet,and in accordance with the well-known relationship between the resonantwavelength of such a resonator and its diameter of cross section, thediameter of cross section of such a resonator would necessarily be aboute6 feet. Obviously, the utilization of an evacuated cavity resonator ofsuch proportions presents constructional and'maintenance problems whichraise serious doubts as to the practicability of the device.

In accordance with the present invention it is proposed to operate acylindrical resonator at a high-order mode designated as the 'IM rn modewhere n is the number of half-cycle variations of the axial electricfield. For the present purpose 1L may be any integer greater than zeroand preferably is of the order of 100. Under such circumstances, aspointed out more fully below, it is not necessary for the electrons totraverse the length of the resonator in a time interval corresponding toa half-cycle variation in the excitation frequency. When excited in theTMO,l,n the standing Wave configuration within the resonator comprisesnodes in the longitudinal component of the electric field, successiveones of which are spaced a distance comparable to, although somewhatgreater than the free-space half-wavelength at the operating frequency.

For modes of oscillation of order in which n is greater than zero, it isknown that the resonant Wavelength is dependent on the length of theresonator as well as on the diameter of crosssection thereof. It may beshown that for a mode characterized by n=l00, the required diameter ofcross section for a resonator of length equal to 30 feet is only severalinches provided the wavelength is suitably chosen. Or, assuming that aresonator of a predetermined small diameter as compared with its lengthis selected for use in the accelerating or energizing apparatus, it hasbeen found that a mode of excitation of sufliciently high order may befound which satisfied the conditions for the maintenance of the desiredfield.

ayeaaati be equal to the distanceibetweemsuccessive nodes in theresonator so thatithe .electrons .mayalwaysencounteriacceleratingelectric forces. 'fFor energies 01 the orderhere contemplated :the velocity oi the electrons :is i veryznearly'zequal toxc thevelocity fofl light. Z'Tnus, r the necessarycondition tobe satisfied maybe-restated inxthefollowing manner: the.distance between successive nodes-ishouldabe equal .:to the free-space"halfwavelength at the operatingfrequency.

Inasmuch as the internodal .distance in the cavity resonate:is':s.omewhat-- greater than the free-space.shalf wavelength and,- sincethewelectrons :cannotxhave: velocities greater than c, itis .cle'ar thatunless t e interncdal distance within thesresonator ..-betreduced tosubstantially the "value :oi' the=-free-space shall-wavelength,electrons :will be subjectedat timestozdecelerating 'iforce :fieldsr aswe1l=as accelerating-force fields, xtherebyeseriously reducing theefiiciency of the apparatus.

As a solution of the foregoing problem, the -.present inventiontprovidesmeans within-a cavity resonator for "precisely fixing spacing :ata valuesubstantially-equal to the free-space half-wavelength thereby insuringthat the internodal the electrons, traveling with a velocity almostequal to that of lightyaresubstantially always in anaoceler-ating forcefield.

--More specificallyas-wlll appear in the detailed descriptionhereinbelow, a preferred form of this feature comprises a typeof-alteration of the electromagnetic field existing :withinthecavityresonator which is accomplished by providing constriations within:the otherwise smooth-wall resonator, the. constrictions being so;arranged as to reducethe diameter of the resonator by .apredeterminedzamount and through-a predetermined lengththereof.Toproduce the desired value of the internodal spacing, the constrictionsaredisposed in the' median planes ofthe longitudinal electric. field, inthe region where the radial comv ponent of the electric field is. amaximumend the axial component .isiia-minimum. A convenient means forproducing such constrictions-has been found tube in the iormofconductive rings mounted within the resonator and extending peripherallytherearound.

It will be understood that it. isvwithin the. contemplation of thepresent :inve'ntion 1 that the energy absorbed by theelectronsismainlymanifest by increased-:massof the electrons. The

changein the velocity, at such high velocities; is practicallynegligible. The overall result mainly resides in theincreased energy-or"the particles.

However, the apparatus of the presentinvention will sometimes becalledranelectron accelerator,"

even when the acceleration :is negligible, in: ac-

- cordance with conventionalusage of .the'aword.

To determine the magnitudezof. .the radio-irequencypower' required toobtain electrons having energies; of electron. volts, :let:lusazconsider... a

simple...numerical1 example. We have previously foundithat a cylindricalcavity resonator excited in the TM0,1,0 mode twould have .an approximatelength of 10 centimetersatotprovide operating frequency, .as discussedabove.

ehavingithezaboveespecifiedwenergy- This results Efrem :theconservative:estimate of the possible voltage .;gradient (10 volts 1 percentimeter), .WlEIlOh.1CB.ILbB successfully maintained in such aresonator. "Where the excitation; employed is in the lTMoamimodeyit. isapparent that a similar overall length of resonatorwould lee-requiredfor :the'production ofelectrons having a billion electronzyoltsef.zenergy. ,If theexcitation energy .zsupplied to tuber l has, awavelength equal to .120 ;cm., arcavity resonator-.sectioniormed by.conducting-wallspositioned at the points-p an'dg will rlriaveia shuntimpedance:otapproximately -.l-() .aohms .and :willxbe .a.halfewavelengthuor 110 cm. long. ..:riccordinglypeach. of these-1.0.0111;;5881710115 .-.providesl0 volts;andthe peakpower: per; section-;is:..given.:by

1, R 10 Watts pable of delivering power in excess of this quan- .tity.

To obtainelectrons.having the same energy,

. it.is.possible to reduce the requisite amount of ex-..citation.energy.by .the simple expedient of employing. alongertube'tl. Withinlimits, the re quired powerandtube length areinverselypro- .p-ortional. At the same operatingfrequenoy electronshaving the specified energy are. obtainable with atube length of 1& cm.or approximately ..300.ft. with .an input power of kw. "l/Vith a.constant tube length, the power required varies .substantiallyasthesquare of the electron energy. .Electrons having an energy ofapproximately 19 .eleotronvolts are obtainable with a 30. it. tubewithan averagepower of 10 kw. or a peak power of 10 megawatts.

Theabove calculations includecertain approximations. Inarriving at theshunt impedance,

.copper' losses in the assumed end walls of each .10 ...cm.-section wereincluded, whereas these .-.losses..wo.uld not occur with thecontemplatedcavity. resonator such as tube 2! having a. length equal to numeroushalf-cycle variations at the Furthermore, no account was taken of thefact that the electrons are not exposed to the peak value of the..electriofield all of the time in their passage through the tube 25.However, these two errors tend to cancel each other.

The following ,radditional illustrations provide an indication. as tothe approximate interrelationship of the various-design factors. It isassumed that the'excitationenergyhas awave- *lengthof 20 cm. and theaverage power, based ona pulsing time natioof 1000 to 1, will bediscussed. Eleotrons of 100 in. e. v. are obtainable -.with a 30 ft.resonator operating with a. voltage gradient of 10 volts per cm. andrequiring l0 kw. It'lwilLbe noted that this arrangement produces.anzenergy equal to that. of thelargest existing .f-betatron andcould'be constructed at-a frac- :tiOlLflfqthfl cost of the; latter.Tor-procure an ener y D I electron voltsatube .lengthof 3,00

9 it. maintaining a voltagegradient of 10 volts :per cm. with a requiredpower of 100 kw. represents at the present state of the art the bestcompromise between tube length and input power. For a relatively smallinstallation a tube length of 3 ft. having a gradient of volts per cm.with a 1 kw. power input provides an energy of 10 m. e. v. Such anarrangement has numerouscommercial possibilities, such as an industrial-X-ray equipment for photographing holes in castings and the like.Moreover, inasmuch as such a resonator is only three feet. long it canreadily be sealed off and the combined equipment will weigh but a fewhundred pounds.

In Fig. 2 there is illustrated one embodiment of the invention togetherwith a schematic showing of the energizing circuit associated therewith.A wave guide 2! partially enclosed at both ends with apertured discs 28,28 and made preferably of copper, copper-plated steel or brass providesa suitable cavity resonator. The internal surfaces of the resonator thusformed are made highlyconducting to minimize power losses in the device.Throughout the length of the guide 1 constrictions may be arranged inthe form of annular rings 29 located normally with respect to thelongitudinal axis of the guide 27. To avoid spurious field emissions,the inner edges 3! of the rings 29 are preferably rounded. Thecenter-tocenter spacing of successive rings 29 is made equal to ahalf-cycle of the free-space wavelength at the operating frequency sothat numerous half-wave sections 32 are formed for the purpose discussedabove. At each end of the guide 21 an end section 33 is formed on oneside by a ring 29 and on the other by an apertured disc 28. A source ofelectrons 34, providing a beam which is concentric with the guide 21, issupported within a glass bell or press 36 by a lead-in strut 31. Theglass bell 36 is, in turn, connected by a met'al-to-glass seal 38 to theend of guide 27. Similarly, at the other end of the guide 2'! anotherglass bell or press 36 isconne'cted with a suitable seal 38 tofacilitate the maintenance of the internal vacuum of the assembly.

The electron gun 34 may include the following elements (not shown): acathode with an electron-emitting surface, an indirect heater and afocusing or current control'electrode.

For'the control and supplyof energy to the equipment, a generator may beemployed having the desired output frequency and power intensity, suchasa magnetron oscillator 42, pulsed by a pulse generator 43, which is alsoconnected to a delay line 4 3. The delay line 44 has two leads 46, 41,connected to the cathode lead-in strut 37 and the outer wall of theguide 21, respectively. A coupling device such as an input wave guide 48having avacuum-tight seal 39 is employed to transmit energy from theoscillator 42 to the .guide 2?. In place of the input wave guide 48, aconcentric line may be used,l in which case it :should be properlyformed at the point of con- :nection to the guide 21, in a couplingloop, probe or antenna means to effect a transfer of energy to the guide21.

' It is, of course, understood that several oscillators, depending onthe desired power input to the guide 27, be arranged in parallel toaugment the energy supply to the input wave guide 48 or other couplingdevice. Moreover, a plurality of coupling devices making connection withthe'guide 2'! along its length may .be fed by one or more oscillators.-Where a plurality of coupling devices are used, care shouldbe-taken to10 insure that the proper phase relationship among the energy introducedinto the guide 21 is maintained. To achieve correct balance in thisrespect, suitable phase shifting apparatus may be employed inconjunction with the various coupling devices.

Where the production of X-rays is desired, a suitable target positionedin the path of the highenergy beam of electrons may be employed. Forconvenience, an X-ray producing medium, such as target 39 may be mountedwithin press 39 by means of a supporting member 41. The physicalorientation of the target 39 may be selected to provide the mosteffective utilization. If desired, a positioning mechanism (not shown)may be used in conjunction with the target 39 and associated parts topermit the altering of the position of the target 39 during orpreparatory to the operation of the apparatus.

It is, of course, understood that, in the event it is desired to employdirectly the high-energy beam of electrons for medical therapy, or otherpurposes, not only the target 39 but the bell 36 may be dispensed with.In such case, to maintain the internal vacuum, the aperture in end wall29' may be covered by an electron-permeable substance or the entire endwall may be made of such substance. A light metal, such as magnesium,aluminum or beryllium constitutes such electron-permeable substance. Inthis connection it should be noted that the electron beam when passingthrough the exit end of the guide 21 is possessed of an accumulation ofconsiderable energy.

However, whereit is desired to obtain a fine control of the energy ofthe exit beam, both the type and thickness of material may be selectedto alter the magnitude of the energy of the exit beam. Indeed, such asubstance interposed in the path of the exit beam may be employedindependently of any'sealing off of the end of guide 7 and solely forenergy control purposes. In addition, the entrance end of the guide 27may be modified by eliminating the glass bell 36. In its place anelectron-permeable substance may be used to cover the aperture of endwall zfi'with a source of electrons positioned adjacent the inpute endof the guide 21.

In operation, a beam of electrons emanating from the electron source 34is periodically projected through the apertured disc 28 and made totraverse the longitudinal axis of the guide 21.

Upon entering the guide 21, the electron beam is subjected to theaccelerating electromagnetic field maintained therein. With theinternodal spacings of the field pattern being maintained substantiallyequal to the free-space half-wavelength at the operating frequency,electrons entering the guide with a velocity approaching that of lightwill traverse each half-wavelength section 32 in a time interval equalto the half-cycle variation in the electromagnetic field. As a result,the electrons will always be present in a force field favorable fortheir increased energy, as previously described in connection with theintroductory remarks and Fig. 1.

In order to reduce the power required for the operation of the device, apulse generator 43 is employed to control or activate the supply ofelectromagnetic energy from oscillator 42 to the guide 21. At the sametime, the pulse generator is employed for activation or energizing thecathode circuit through a delay line 44. Thus, electrons emanating fromthe electron source 34 are injected into the guide 21 periodically andat aesaaan such times when the voltage of the electromagnetic fieldcontained thereinahasattainedja sufe ficient magnitude. The attainingofsufficient' electromagnetic voltage within-the gui'deizil prion to theinjection OfithB electronsds achievediby reason of the delay line 44interposedzbetween-the: electron source 3t and the pulse;generaton'dfii? While in preceding paragraphs.it;has.;been :in= dicatedthat a right; circular: cylindrical waveguide is eifective=for.-propagating an electric node. of the type TMmmwhich type-ofnode=is=par-- ticularly suitable for the acceleration of:,e1e,ctrons, it is of coursetunderstood thatthexwave energy guide zlrma-y.assume other shapes with-:- out departing from the: spiri-toftheninvention':v Furthermore, apart from the conductive annular rings29, the wave energy guide 21, 96 :(the latter in connection with Fig.11) has been described as being hollow, however, dielectric material maybe used in part to obtain desired electrical characteristics. Thus, sucha dielectric material may be used to fill merely aupartofthe-guideor thewhole thereof.

In the introductory remarks itahaslibeenindicated that the electrons maybeenergizedby passage through a resonator. It is, of course; understoodthat the aoceleration'of the electrons. may be achieved by projectingthem through a traveling wave electromagnetic field. In such a;

case, electrons should be made to ride-thewave crest or, stated somewhatdifferently, have. a: phase relative to the phase of.theelectromagneti'c field such that they are continually.positioned forthe absorption of energy from the field;

It will be noted that there is asection 33 havin a length substantiallyequaltotheiree-space quarter-wavelength at the operating frequencylocated at the input end of the guide 21; In the event that theelectrons emanatingfrom the electron source. 34 are injected with somevelocity relatively low-with respeet-to'that of lightpand which isproperly relatedtothe level of the radio frequency excitation power; theaverage-velocity of the electrons during the first quarter-wave-= lengthmay be approximately one-half thevee. locity of light. As a consequence,the electrons will arrive at the first node at a time when thecyclically-varying electromagnetic fieldreverses. Moreover, where themagnitude ofthe-radio frequency energy employed-is substantially high;the electrons will arrive at thefirst node: with a'velocity very nearlyapproaching= that of Wight; Once such a velocity is attained by theelectrons, there will occur relatively-small velocity changes; so thatuniform loading, providedhysuitably positioned constrictions maybe usedthroughout the remainder of thelength of theguide-Tli Theseconsiderations are equally applicable to any of the other modificationsof the presentin vention described herein.

In Figs. 3a and 3b. graphs of the volta-ges ex* isting at various pointsin the circuit of Fig. 2 are illustrated. In Fig. 3a thereisplotted awave form or the voltage appearing at the output ter-' minals of thepulsegenerator 43.- Asa resultof' the connection of the pulse generator-43Cto the magnetron oscillator 47., the output wave-formnf theoscillat-or has the form illustratedby Fig. 3b? As previouslydescribed,owing to the: delay'linewhich is interposed between the pulsetgeneraton3 and the electron source-34; electrons-are not projected through thewaveguide 21' until the excitation voltage therein-has assumed asuficient amplitude. In this connection an explana tory numericalexample mayprove useml As 1 2 previously discussed; .withatime ratio of1000 to l,iit:is:possible to obtainelectrons having the desired energywithout employing an excessive highspowerz excitation. energy source. Inthe events that: the' time duration of each pulse is madeiequalitoone-microsecond, with an excitation" wave having a-frequen'oy of 15x10cycles perrsecond; therewilloccur approximately 1500 cyclicalvvariations ofxthe electromagnetic field forreachpulse Novattemphhaswbeen made to show thexbuilding; up; of the-ivoltage of oscillator 42 itto. its. normal: operating j output: voltage, norexcitation:voltage;-withirr.the:i-guide27. With a .05 microsecond.delayr introduced by: delay line 44,15cyclica'1Nariationsiwillloccur':(based on the above-mentionedoperating-frequency and pulse durations) prior: to .theinjection ofelectrons into the guide 21. Thisdelay will compensate for boththeivoltage; Off. the: oscillator 42' as well as the f radio frequencyexcitation voltage withinthe excitationenergy.suppliedpto the guide 2'!during each? pulse; will'necessarily be lost'as far as chargedpartioleenergization is concerned. I-Iowever,:this'. is a negligible quantity ofpower which does.not:impair the functioning or the device.

Whilethe above numerical example was based on a timerratio iof .10001to.=1 for convenientrepresentation,..a:ratio of 25'tor1'hasbeen employedin.Figs..3d"an"d:3b;=.

Whereitisxdesired totemploy a smaller magnitude of. radio; frequencypower and a source ofs'relatiyely slow moving-t. electrons, i. e.,electrons whichishave a velocity substantially lower than that of:li'ght prior to'being' introduced into-the energizingvportioniofitheapparatus, a wave guidel'l :may .be .formed with tapered constrictionsas shown in Fig.4; To permit the injectedtelectrons:whichlare:relatively slow movingsto'z attain: substantially thevelocity of light, several sections. 54,.56;;51,:58,3 59 of unequallength are provided adjacentrthe:entrance/end of the guide :2'l;,'ras.indicated by the arrow fi l. Similarly,wherei.the-inputtradio:.frequency power to the guide 2'l is :relativelysmall: in magnitude, the tapering of the successive lengths of theseveral sections .54.; 56, 51,53 ,159: may be advantageously employed;Thei-sections 54;: 5t, 5?, 58, 59 are formed with tapered annular ringsti, 6! having increasing centen-to-centert spacings along thelengthzofrthe. guide 21 .until 'the axial extent of the'lastisectionnfiz isa substantially equal to the free spacehalf-wavelength at the operating frequency whereupon the constrictionsare arranged as described inconnectiorr with the. major portionsof theguide 21 of Fig-2-01" Fig. 3. The initial-section formedat the entranceend of the-sleeve121by endiiwall' 28 andlring 0| has a length somewhatlessthan one-half the centerto-centerdistance betweenarin'gsi 6 I, E I

Preferably, the. inner diameterof the tapered ringsli i, 6 IL: isselectedto. be progressively larger in the direction of the electronflow; In addition,.the:width"of. thetapered ringsel, 6| may be madeprogressively larger along the length of theguidell. As discussed prviouslyin connection with the taperedrings 6!, 68, the inner edges .63may be rounded to discourage field emission.

In'operation, a beam of' electronsis made to traverse the longitudinal"axis of the guide 2'5 in the directionindicated' bythe' arrow '04.Where relatively slowly injected electrons and where relatively lowradio frequency excitation power are employed, the progressively largerlongitudinal dimensions of the sections 54, 56, 51, 58, 59 permit theelectrons to increase in velocity. To this end, the respective lengthsof the sections 55, 57, 53, 59 are selected so that electrons passthrough an individual section in a time during which the field isfavorable for providing acceleration or gain of energy of the electrons.A transit of one of the sections 54, 55, 51, 58, 59, 62 having beencompleted, the electrons move into the adjoining section 55, 55, 5'3,58, 59, 52, the electromagnetic field of which is suitable for impartingadditional acceleration. In this manner an electron is carried alongfrom one section to the next until it has attained a velocityapproaching that of light.

Care must be taken to achieve the correct spacing of the successivesections 54, 55, 51, 58, 59, 62 as a function of the initial electronvelocity and the imparted acceleration due to the excitation radiofrequency fields. Otherwise, an electron may not enter a section 54, 55,51, 51, 59, 62 at a time favorable for extracting energy from the fieldand, in fact, may do work on the field, i. e., deliver some of itsenergy to the electromagnetic field with a resulting loss of energy ofthe electron. In the ideal case, the electrons will substantiallytraverse each section in a time corresponding to the half period of theexcitation energy.

A symmetrical coupling arrangement for introducing radio frequencyexcitation power into the apparatus is shown in Fig. 5, which is amodified form of the device of Fig. 2. The excitation energy is coupledto the device at the entrance terminal thereof, providing for thesymmetrical coupling of radio frequency power into the apparatus at theend adjacent the electron source.

As described in connection with Fig. 2, a wave guide 2! having highlyconductive internal surfaces is divided into sections by means ofconstrictions which are formed by conductive annular rings 29. With theexception of the end sections 33, the spacing between successivesections is substantially equal to the free-space half-wavelength of theexcitation frequency. At the left end of the guide 27 an apertured disc23 having an internal surface of highly conductive material is provided,thereby forming a freespace one-quarter wavelength section 33intermediate the inner surface of the disc 28 and the adjacent annularring 29.

A hollow cylindrical housing 56 coaxial with the guide 2? is connectedin an air-tight manner to the disc 28 and forms an enclosure for thesymmetrical radio frequency coupling device. For coupling energy to theguide 21, cylindrical coupling sleeves or antenna 5'! located coaxiallywith respect to the guide 2'! is arranged preferably extending partiallyinto the adjacently located quarter-wavelength section 33. To 'energizethe antenna Bl, a coaxial transmission line having an inner conductor 68and outer conductor 59, the inner conductor being supported by aone-quarter wavelength shorted stub H, is arranged extending radially tothe guide 27 and the coupling sleeve fi'l. One end of the outerconductor 59 is connected to the edges of a suitably formed aperture 12in the housing 56, with the inner conductor 68 connected to andsupporting the antenna 51. The other end of the concentric line 10 isconnected to asuitable source of radio frequency energy as shown and.described in connection with Fig. 2.

While only one coaxial line 10 coupling device is shown, it is, ofcourse, understood that several such lines might be employed. In suchcase, it is preferable to maintain a symmetrical disposition of thelines feeding the antenna 51, in order to preserve the appropriate phaserelationships. However, if unbalance occurs in this respect, adjustablephase shifting devices (not shown) may be included in the coaxial lines.

An electron gun 34, similar to that previously discussed in connectionwith Fig. 2 is supported from a glass bell or press 13 by a lead-instrut 37.

To provide for the maintenance of an internal vacuum, the glass bell 13is connected by means of an air-tight seal 14 to the end of the housing66 and forms with a portion of the latter an enclosure for the electrongun 34. Intermediate the antenna 61 and the electron gun 35 an electromagnetic field terminating diaphragm "I5 is located normally of thetube 66.

For certain applications, it is important to operate the apparatuswithout the excitation of certain modes or types of electromagneticfield patterns in the guide 21. A particular advantage of thesymmetrical coupling arrangement described above is that certainundesired modes are not excited, that is, those modes which do not haveaxial symmetry of the electric field.

In operation, a beam of electrons emanating from the electron source 34is periodically projected through the aperture of field terminatingdiaphragm 16 and through the coupling sleeve 51. Upon entering the guide2'! the beam of electrons is subjected to the radio frequency fields andis accelerated or energized in its transit therethrough in the mannerpreviously described in connection with Fig. 2. The diaphragm, whilepermitting the passage of electrons from the electron gun 34 to theresonator 21, minimizes the loss of the excitation energy endwise of theguide 2'! in the direction of the press 73.

A further modification of the device of Fig. 2 in which the excitationenergy is introduced at the terminal end of the Wave guide 2'! is shownin Fig. 6. This arrangement has the advantage of the symmetricalcoupling of radio frequency energy into the guide 2'1 described inconnection with Fig. 5 and at the same time provides a somewhat simplerand more economical construction. As before, the internodal spacings oithe field in the guide 21 are made substantially equal to free-spacehalf-wavelength at the operating frequency, An apertured disc 25'together with an adjacent annular ring 29 forms a one-quarter wavelengthsection 53 at the exit end of the guide 21. For introducing theexcitation energy a coaxial transmission line TI having an innerconductor 78 and an outer conductor I9 is provided, with collar 85,which is rigidly attached to disc 28', supporting line H. It isunderstood that a suitable seal will be employed between the collar 83and outer conductor l9 of the coaxial line 11, and between innerconductor 18 and outer conductor 79, in order to preserve the internalvacuum of the apparatus. A probe antenna 8! which is an extension of theinner conductor 73 projects into the quarter wavelength section 33 atthe exit end of the guide 27 and is concentric therewith. To support theinner conductor 18 a dielectric ring 82, having suitable dimensions toprevent spurious reflections of energy, may be provided. Moreover, as anal- 15 ternative to the. dielectric rings; 8240.1: toprovide additionalsupportrioi'. the innerconductor iiia one-quarter wave shqrt-circuitedstubv (not shown) may be employed.

To offer a negligible obstruction: to the high energy electrons theantenna 8! and concentric line il perferably may, be made of some'lightmetal, suchas magnesium, aluminum, beryllium, etc. Similarly, if,desired, for reasons of manufacturing simplicity; or. otherwise, thedisc 28 may be made of these, materials.

If the electrons emanating from the terminal end of the resonator 21 areto-be employed for the production of X-rays, a target of suitablematerial may, be positioned along the longitu dinal axis of the guide 2?beyond the disc 28.

The above described symmetrical coupling may be employed in, conjunctionwith the coupling arrangement, shown in Fig. 5, where an additionalamount of excitation energy is required. In such a. caseprecautionarymeasures must be taken to obtain the proper phase relationship betweenthe energy supplied both at the input andoutput end'of the guide Z'l.This proposed arrangementhas all the advantages of the symmetricalcoupling and atthe same time provides for introduction into theapparatus of an increased amount ofexcitation energy.

The operation of the apparatus shown in Fig. 6 is substantially similarto that previously described inconnection with Fig. 2.

In Fig. '7 there is shownavv further modification of the presentinvention in which the excitation energy is coupled to'the apparatus ata plurality of points. As previously discussed, the internodal'spacingof the electromagnetic energy contained within the wave guidefl isreduced by the use, of suitably disposed rings 29 which preferablyhavetheir inner edges 3i rounded. For uniformly. distributed excitation,a coaxial transmission line 8 1 having an inner conductor 85 and an,outer conductor 81 isarranged adjacent to the guide 27 alongits length.In order to support the inner conductor 86 of the coaxial line 84,quarter wavelength shorted stubs (not shown) or other similar means maybe employed.

To introduce radio frequency power from the coaxial line to-theresonator, coupling loops 88, 89, 99, 9I are provided which extendthrough apertures common to walls of the outer conductor t1 and theguide 21'. Theends of each coupling loop are connected to the innersurface of the, wall of the outerconductor 8i and the inner surface ofthe wall of the guide'2'i', respectively. The sizes of the couplingloops 88-91 are preferably increased along the length of the guide 27from its entrance terminal to its exit terminal, to minimize thereflection of energy in coaxial line ti l, as discussed more fullybelow. Similarly, the apertures-associated with the coupling loops 88-ill may be progressively enlarged to accommodate their respective loops.

It will be noted, as shown more clearly in Fig. 8, that a portion of thewalls of the outer conductor 81' and the guide 2"! overlap along theirlengths in the vicinity of the coupling loops lit-9i.

In operation, coaxial line 84 is connected at one end to a source ofradio frequency energy, such as the magnetron oscillator 52 shown inFig. 2. With impedance matching existing between the oscillator 42 andthe coaxial line a l power losses in the line 84 are'negligible and anextremely low standing wave ratio is present therein. The coupling loops83-9l are made progressively larger throu houtlthe axial? extent of thetrans.- mission; line fi t thegsize-oi each; loop being selectedsothatit extracts a predetermined amount of power from the line 84; Morespecifically, the the, dimensions of the individual loops are chosen sothat a constantly increasing percentage of the power remaining inv thetransmission line til is introducedinto-theguide 27 by each successiveloop $34M. In this manner; a substantially equal amount of power isintroduced into the guide 21" by each of the loops 88 through ti and,moreover, all of the, radio frequency energy is extracted from the, line84- prior to its terminal end. Asa result, there issubstantially noenergy which maybe reflected backthrough the transmission line 84 in thedirection of the electromagnetic energy source and, hence, standingWaves are substantially avoidedin the line a l.

Apartfrom the specific arrangement for introducing the radio frequencyexcitation energy into the guide 21, the operation of the device issimilar to, thatdescribediin.connection with Fig.

Of course, it is understood that the coaxial line 84 does notnecesarily. haveto extend-for the full lengthof theguide 21', but may atany convenient point be made to extend directly to the electromagneticsource.- Such proposed lines may merely be effective for energizing asingle or but a few coupling loops, such as the loops 38, 89, 9B. Other,transmission lines arranged in the manner of linet tmaybe employed toexcite distinct groups of coupling loops. As a result, a plurality ofcoaxial line; feeding sections such as that shownin Fig. 7' may beemployed with a single or a plurality ofihig-h frequency generators. Inall of the foregoing versions suitable phase shifting apparatus may beadvantageously em ployed in the respectively feeding lines to obtain theproper phase relationshipamong the-input energy.

The distributed input coupling arrangement of Fig. 7 may. also bemodified by employing a plurality of coaxial lines arrangedlongitudinally with respect to the guide 2? shown in Fig. '7 and spaceduniformly. around the periphery of the guide 27. This arrangementprovides not only for the uniformly distributed introduction of,excitation power into. the guide 2?, but in leltddition. achieves asymmetricalexcitation as We In- Fig. 9- there isshown a modification ofthe arrangement of Fig. 7 which also achieves a distributed coupling ofthe excitation energy to the guide 21'. This embodiment diiTers from theshowing of Fig. 7 essentially in'the omission of the-coupling loops'andthe, utilization of coupling slots 92, 93, 94 having progressivelylarger dimensions in the direction of energy propagation along line 84for efiecting a transfer of excitation energy to the guide 27'. Thewalls of the guide 21' and the outer conductor 8'! of the coaxial line84-aremadeto overlap in the vicinity of the slots; 92, 93, 94 asshownmore clearly in Fig. 10.

The operation or this device, .aswell as possible modifications ofthestructure, is similar, to those described, inconnectionwith Figs. 7and 8.

lf'he distributedcoupling disclosed in connection with Figs. '7 through10 has at least three important advantages: (1) The starting transientat the beginning; of each. pulsedoes not have a long duration comparedwith that of the single coupling. arrangements. (2') Undesired modes ofthe electromagnetic field are suppressed to an even. reat r extentv th nwith the coup i compartments H13, 4434,

1(3) I he ma 92 throng-11 94 is substantially less than with fewer:coupling units, which is 'an important 'fac- 1 itor a'trhigh powerlevels.

Theiprefer red embodiment of the present in "vention, 'vvhich attainsmaximum "electron -e'n- *ergy with." great power economy through the use'of a symmetricail-distribution couplingiof theira- .110

-'-dio frequency into the'wave-guide, tapered loading and synchronous:pulsing :of the excitation energy and the electron source, isiilustrated in --Fi'g. 11. in this'arrangement the advantages of boththe symmetrical and distributive apparatus in introducing the excitation:power into the resonator are achieved. Moreover, --1oading ='featurepreviously 'described in connec- 'tion with Fig-ki is advantageouslyutili'z'ed'so-that the '-tapered electrons having initialvelocities-substantially less than the velocity of light may beemployedas well as -iowerexcitation energy. Furthermore; provision is made forpulsing both the input energy to the resonator and the electron source,

"such pulsing being particularly effective for ob-i --taining ahighenergy charged particle beam.

A wave (guide .96 is provided with constrictions which reduce theinternodal spacing of T the-electromagnetic field, as previouslydiscussed.

However, the guide 95 is constituted of a plurality of energy-directingsleeves 91, 98, 99,1111,

M32 which divide the guide into a plurality of I95. Each compartment493, Hi l, titis-constituted of an energy- -directing 'sieeve 91-98, 99,-i-fii,wl-02 with an en- .larged diameter portion 181, 198,1139, Ill,H2,

respectively, which extends over the sleeve '01? the :adjacentcompartment without making physical contact therewith, to provideradially and axially extending annular slots H3, H4, H6, H1,

418. Theseslo-ts H3, 1 I L'HS, I ll, I [-8 are preferably made .to haveprogressively :lower capacitances in the direction ofelectromagneticenergy propagation so that progressively larger percentages-of the powerremaining in the enlarged coaxial supply line i It may be coupled to thewave guide 9 5. "These annular capacitances may be made progressivelysmaller by progressively increasing the spacing or progressivelydecreasing the efiective areas in the annular capacitance regions, orboth. n v

To support eachcempartrnent its, 1&4, -li.6.,-a plurality of radiallyextending rods 12'! made of .anappropriate material, such has ofdielectric composition are arranged intermediate the outer .wall ofthesleeves 9198,99, lfll, IE2, and the' inner wah o'ft'he enlargedouterconduCtorJl-ZZ.

the wall of the outer conduetorlf'z toaccommodate screws 52d iorsecuringone end of the rods i2i3'th'ere'to. "The "other endl'of the rod's-l'23are fflared with their end surface suitably-raced for Toooperating withthe outer Wall of the sleeves 93,,"394 $81., $552.. A Wave-directingend-Sleeve 12-6 "is connected with the inner conductor .IfZTof theenlarged outer conductor 1 22 of the fenlarged coaxial supply line H9with the constricted outer conductor :53! of the constrictedcoaxialsupply line; 8., tin-cuter conductorconic'al. portion I 32"Suitably located apertures 123 are provided in "by the wave guide 96.For'i-nterconnectin'g the,

"of theendsl'eeve F26,'the'conical1portion i32 being fcon'nected 'to anintermediate outer "conductor I portion [35 .WhiCh iSWGflXifil with thehollow inner conductor spor'ition iil ifin order to supply excitationenergy to the device, the :coaxial supply line =i-2sis connected to anexcitationenergy source, sueh as a magnetron oscillator *42, th'e outputof which is conitr'o'iled by a pulse generator i3. To provide a "sourcerdf e'i eetrons, which are to 'be accelerated, iian fe'lectromsource E iis arranged 'ext'erior to both othe enlarged ic'caxial supply line H9,which houses theiwave guidett, and the reduced diameter-supply line IE8.The electron source, which "contains suitable electron-emitting surfacei-vlith I appropriate potential-fixing elements (not shown)-, is alignedto insure that the electrons 'arezlprejected successively through anaperture 13B the constri'cte'douter conductor [3 1, the hollow innerconductor portion and thence along the axis'of the wave guideIn-add'ition to controlling 'the'outp'ut of the magnetronoscillato'r 12,'the pulse generator 43 regulates the introduction of electrons int-othe waveguide 396. It is, of course, understood that electrons maybeintroduced into the apparatus from the opposite end. However, with "thisarrangement the form of 'the co'nstriction-s will be somewhat different,

as discussed below.

"In order to reduce the internodal spacing of --the electromagnetic wavecontained within waveenergy guide -96, constrictions in the form of-annular rings are arranged throughout the tength of :guide "96. Thecompartment formed 0f the end sleeve [-25 and compartments "I33 and Add,:arecprovided-withannular rings 6|, 61", 433 in a manner similar to thatpreviously described in connection with Fig. 4. Thus, the rings fil, 64i'33 :have "increasedcenter to-centerspacings, --aperture diametersand'widths in the direction of electroma-gnetic energy propagation. Toaccommodatethe variation indimensions of the -rings -6l,,-6-l' I33,the-associated sleeves 126, '91, -;98, respectively, have appropriatelongitudinal --dimens-ions. in the device of Fig. 11, there -has-been-sh-own'a plurality of compartments 1413,4434,suitahleforobtaining thetapered loading teaturewhich wasde'scribed more fully in connection withFig.4. =Itwil1 be, of course, understood that where desired, a "greaternumber of such compartments W3, H14 may be utilized. Where it is-desiredto inject electrons from the 'uppositeend ofthe guide -96, the taperedloading rings 61-, 51.2;133 will be arranged in the opposite order isothat the increasedcenter to-center spacing-of the rings 51, '51, I33will occur in the direction of the path of the electron beam. With suchan arrangement, the electron source 34 will be positioned at theopposite end of the guide '96. Conrpartrr-rents wt-are formed 'withannular ring id, which asirniiar-ly-reduce the internodalspacing;-of;the;electromagnetic energy contained .within theguide -96.Assuming that the electrons travelling longitud-inally through the guide'96 have obtained a velocity substantially equal to thevelocity-oblig-ht, the Fannula'r rings '29 are arranged in such amanner-as to provide inter- 330K185. spacing substantially equal to thefreespace half-wavelength eat the operatin'g frequency. in-this manner,the desired highlyen'er- :g-ized beam-oi charge'd particle's isobtained.

To "insure ":that the internal "vacuum or the apparatus is maintained;a- 'gla's's' be1114'lsurrounds the enlarged outer conductor I22, apernects the outer conductor I3I and the glass bell I4I. Additionalseals (not shown) may be used to provide a vacuum-tight connectionbetween the wires I44 and the glass bell I4 I.

Conventional devices, such as short circuited quarter-wavelength line orstubs (not shown).

may be arranged radially to the constricted diameter coaxial supply lineI23 to provide support for the inner conductor I27. If desired, one ormore supports or struts similar to those used in connection with sleeves91, 98, 99, Ifil, I02, may be;

employed for providing additional support for the end sleeve E28.Furthermore, suppressors of undesired nodes (not shown) may be includedin the enlarged coaxial supply line H9. An annular dielectric ring I34,I36, I31, I38, I39 of suitable composition is shown included in theannular slots H3, H4, H6, H1, H8, respectively, to increase the powerhandling capacity of the coupling units and provide additional supportfor the sleeves 97, 98, 99, IOI, I62. desired, the dielectric rings I34,I36, I31, I38, I39, may be omitted.

In operation, the pulse generator 43 regulates the supply ofelectromagnetic energy, which emanates from oscillator 42, to theenlarged coaxial.

supply line II 8. At the same time, the pulse generator 43 is effectivefor controlling the introduction of electrons projected from theelectron source 34 into the wave energy guide 95. By reason of the delayline 44 interposed between the pulse generator t3 and the electronsource at ample opportunity is provided for the electro magnetic energywithin the guide 96 to reach the required intensity prior to theintroduction of electrons. The electromagnetic excitation energy, whichis propagated through the enlarged coaxial supply line H9 is coupled tothe wave guide 96 through the annular slots H3, H4, H6, H1, H8. Byemploying progressively larger slot in the direction of energypropagation an increasing percentage of the power remaining within theenlarged supp-1y line H9 is introduced into the guide 96. With thevaried size annular ring 65, SI, I33, electrons introduced withvelocities somewhat lower than that of the velocity of light arepermitted to absorb sufiicient energy from the electromagnetic wavescontained Within guide 96 to attain a velocity approaching that oflight. This action takes place within the end sleeve I26 and thecompartments 03, I04. Once the electrons have approached the desiredvelocity, the constrictions 29' are uniformly arranged, providing aninternodal spacing substantially equal to the free-space quarterwave-length at the operating frequency.

Thus, the arrangement shown in Fig. 11 represents the preferredembodiment of the invention and is particularly useful for obtainingelectrons which have the desired magnitude of energy. By employing thedesired loading, it is possible not only to employ electrons which havevelocities substantially lower than that of the velocity of of chargedparticles permits the construction of an energizer capable of deliveringcharged particles having energy greatly in excess of anything However,if

heretofore known. The novel symmetrical distributive couplingarrangement is particularly effective for reducing the startingtransient at the beginning of each pulse, suppressing undesired modes ofelectromagnetic propagations and imposing a greatly reduced power loadupon each coupling unit. The latter point is particularly significant inapparatus operating at high power levels where voltage breakdown andundesirable heating must be avoided. Thus, the device of Fig. 11 isparticularly effective for achieving the major objects of the invention.

Since many changes could be made in the above construction and manyapparently Widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electron accelerator comprising a source of radio frequencyexcitation energy, a coaxial transmission line connected to said sourceof energy, said coaxial line having an enlarged portion, the enlargedinner conductor portion being hollow and comprising a wave guide formaintaining a cyclically-varying high frequency field, said guide havingan entrance end, a source of electrons located adjacent said entranceend of said guide, a plurality of annular rings located within andconcentric with said guide, and means for projecting said electrons intothe entrance end of said guide, the Width and the axial spacing of saidrings increasing along the length of the guide and the diameters of saidrings increasing along the length of said guide, said spacings of saidrings approaching the free-space halfwavelength of said high frequencyfield.

2. In an electron accelerator, a wave guide having internal conductivesurfaces and having an entrance end, a source of electrons locatedadjacent said entrance end of said guide, a symmetrical coupling meansfor introducing excitation energy into said guide, said coupling meansincluding a coaxial transmission line section having a tubular innerconductor located intermediate said source of electrons and said guideand concentric therewith for the passage of electrons therethrough, saidguide having annular rings with center-to-center spacings correspondingto the free-space half-wavelength at the operating frequency whereby thepropagation of undesired modes of electromagnetic energy issubstantially eliminated.

3. An electron accelerator comprising a source of electrons which are tobe accelerated, a wave guide having one end located adjacent saidelectron source, means for projecting said electrons into said guide, asource of high frequency excitation power, a coaxial line extendingalong the length of said guide and having a portion of one of itsconductors common with the wall of said guide, saidcoaxial line beingconnected to said power source, said common portions of said conductorand said wall having a plurality of longitudinally spaced apertures fordistributedly introducing excitation energy into said guide.

4. An electron accelerator comprising a wave guide having an internalconductive surface and constrictions for reducing the internodal spacingand uniformly'distributed round the periphery of said guide and furtherhaving portions of its conductor walls common with the wall of saidguide, said coaxial line means being connected to said power source,said common portions of said walls having a plurality of longitudinallyspaced apertures, whereby a symmetrical and distributed coupling of saidexcitation energy into said guide is accomplished.

5. A high power electron accelerator comprising a guide formed of aplurality of coaxial sleeves having internal conductive surfaces, eachof said sleeves having an enlarged outer diameter portion at one end foroverlapping the adjoining one of said sleeves to provide annularcoupling slots, an outer conductor surrounding said guide and coaxialtherewith, said outer conductor and the outer wall of said guide formingan enlarged coaxial supply line for supplying electromagnetic energy tosaid guide, said sleeves having internal annular rings coaxial therewithfor reducing the internodal spacing of the electromagnetic energytherein, a portion of said annular rings having increasedcenter-to-center spacings, widths and aperture diameters along thelongitudinal axis of said guide in a portion'of said sleeves, theremainder of said rings having uniform dimensions and spacings in theremainder of said sleeves, a source of electrons positioned adjacent oneend of said guide, an oscillator connected to said enlarged coaxialsupply line for supplying thereto said electromagnetic energy, and apulse generator connected to said oscillator for regulating theintroduction of energy into said coaxial line, said pulse generatorfurther being connected to said electron source through a delay line toinsure the excitation energy attaining sufficient proportions prior tothe introduction of electrons, whereby a high-energy beam of electronsis obtained.

6. An electron accelerator comprising a hollow wave guide having alongitudinal axis, means for producing a stream of electrons andprojecting said stream within said guide along said axis, an outerconductor surrounding said wave guide and cooperating with the outersurface thereof to form a coaxial transmission line, said transmissionline being adapted to be connected to a source of high frequencyelectromagnetic wave energy, and means including apertures in the Wallor said wave guide for transferring electromagnetic wave energy fromsaid coaxial line to said wave guide, said apertures being radiallysymmetrical about said axis.

7. An electron accelerator comprising a hollow wave guide having alongitudinal axis, an electron gun adjacent one end of said guide andaligned on said axis to project a stream of electrons along said axisthrough said guide, a plurality of annular rings located within andconcentric with said guide and spaced along said axis, consecutive ringsalong the initial portion of the length of said guide being ofincreasing inner diameters, coupling means including a tubularconductive member coaxial With said guide and located between saidelectron gun and said end of said guide, a source of high frequencyalternating current energy connected to said coupling means, and a pulsegenerator connected to said alternating current source and to saidelectron gun to energize said source and said gun periodically.

8. An electron accelerator comprising a hollow wave guide having alongitudinal axis, an electron gun adjacent one end of said guide andaligned on said axis to project a stream of elec trons along said axisthrough said guide, coupling means including a tubular conductive membercoaxial with said guide and located between said electron gun and saidend of said guide, a source of high frequency alternating current energyconnected to said coupling means, a pulse generator connected to saidalternating current source and said electron gun to energize said sourceand said gun periodically, and a delay line included in the connectionbetween said pulse generator and said electron gun, the delay of saidline being approximately equal to the time required for anelectromagnetic field to build up in said wave guide to substantiallythe full intensity to be produced therein by said alternating currentsource.

JOHN R. WOODYARD.

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